The present invention relates to voice communication systems in general, and more particularly, to a voice communication system utilizing a voice activated switch which is particularly suited for a communication system for divers and aviators.
The use of voice activated transmit switches (VOX) in communication systems is well known and is desirable because it eliminates manual switching from the receiver to the transmitter. In such systems, the normal voice level output from a speech microphone is used to actuate a circuit which enables the transmitter while at the same time disabling the receiver. The most common method of enabling and disabling is to transfer operation power from the power supply from the receiver to which it is normally connected to the transmitter when the voice actuated switch is keyed.
This type of system is highly susceptible to false keying of the transmitter by non-speech noises because the voice actuated switch usually does not distinguish between non-speech signals and speech signals. The problem of false keying of the transmitter is particularly acute in an underwater communication system where the microphone is located in the diver's mask and therefore exposed to the diver's breathing noises and the gas flow noises. In fact, the loudness of this breathing noise inside the mask, particularly when the diver is working hard, is usually of the same order of magnitude than normal speech noise so that breathing would certainly cause false keying.
One method proposed for eliminating false keying in the use of a voice band-pass filter in the microphone circuit to eliminate non-speech noises, but since breathing noises and gas flow noises have a broad frequency spectrum which extends well into the speech spectrum, such filters seriously degrade the quality of voice communication which is already of a lesser than desired quality because of the special environmental conditions to which both the diver and the equipment are exposed to when underwater.
Another solution proposed to the above-stated problem is the utilization of a separate tissue-conducting (throat) microphone, in addition to the speech microphone located in the mask, which is placed tightly against the throat. It is well known that breathing sounds are not normally heard in the throat so that the output of the throat mike is entirely due to speech and may be utilized to activate and sustain the voice operated switch. This system is disclosed in U.S. Pat. No. 3,746,789 which issued on July 17, 1973. The disadvantages of utilizing a separate tissue conducting microphone to operate the voice actuated switch are several, one being the cost of an additional microphone, particularly of the throat type, another and more serious one being the inconvenience of maintaining a throat mike in tight contact with the throat of the diver when even the slightest pressure on the throat may interfere with normal breathing of the diver. A further disadvantage is that the throat mike is one additional piece of equipment a diver has to wear and to maintain in addition to many others.
It is therefore a primary object of the present invention to provide a voice activated switch which is not keyed by normal environmental noise, such as breathing and gas flow, such as is normally found inside the diving mask of the scuba diver or perhaps in the oxygen mask of a high altitude aviator, but which still permits communication using normal level speech.
It is a further object of the present invention to provide an improved voice actuated switch which is bilevel in that it requires a high voice level to key it on and only a normal voice level to sustain it and which cuts off when the input is substantially below the normal voice level.
It is another object of the present invention to provide a voice actuated switch which is particularly useful in an environment where it is subject to noise at a level equivalent to that of normal speech, but in which the noise can be substantially reduced or even eliminated for at least short times.